A burner is one of the most important parts of a combustion system. The performance of the burner not only greatly influences the combustion efficiency, but also closely relates to the stability of combustion flame, the effective utilization of the fuel, and the discharge of pollutants. Improper combustion methods and improper selection of burners not only influences the effective use of energy, but also results in air pollution due to emission of large amount of hazardous matters such as NO.sub.x because of undue combustion.
In order to improve the performance of the burner while decreasing the amount of NO.sub.x formed in the combustion process and increasing the stability of the combustion flame, it is necessary to reduce the peak temperature of the flame, to control the residence time of combustion gas, and to form local fuel-rich combustion.
The dramatic effects of swirl in reacting flow systems have been known and appreciated for many years. Some effects are favorable, and the designer strives to generate the required amount of swirl for his particular purpose; other effects are undesirable, and the designer is then at pains to control and curtail its occurrence. In combustion systems, the strong favorable effects of applying swirl to injected air and fuel are extensively used as an aid to stabilization of the high intensity combustion process and efficient clean combustion in a variety of practical situations: gasoline engines, diesel engines, gas turbines, industrial furnaces, utility boilers, and many other practical heating devices.
Swirling flows result from the application of a spiraling motion, a swirl velocity component (also known as a tangential or azimuthal velocity component) being imparted to the flow by the use of swirl vanes, by the use of axial-plus-tangential entry swirl generators, or by direct tangential entry into the chamber. Experimental studies show that swirl has large-scale effects on flow fields: jet growth, entrainment and decay (for inert jets) and flame size, shape stability and combustion intensity (for reacting flows) are affected by the degree of swirl imparted to the flow. This degree of swirl is usually characterized by the swirl number S, which is a nondimensional number representing axial flux of swirl momentum divided by axial flux of axial momentum times the equivalent nozzle radius. That is ##EQU1## where ##EQU2##
Swirl flows are generated by three principal methods--
1. tangential entry (axial-plus-tangential entry swirl generator) PA1 2. guided vanes (swirl vane pack or swirler) PA1 3. direct rotation (rotating pipe).
Among these methods, commercial burners have tended to adopt the guided vane system, where vanes are positioned so that they deflect the flow direction. A conventional burner usually feeds air into the combustion chamber by means of a fan or a compressor and using blades of fixed radial type to mix the air with the fuel for burning. However, it is found that the performance of the burner will become very poor if the pressure drop is too large and the turbulence intensity is too high when the air flows through the fixed radial blades in the conventional burner for forming swirling flow, which is required for combustion.